(a) Field of the Invention
The present invention relates to a display panel, a light emitting display device using the same, and a driving method thereof. More specifically, the present invention relates to an organic electro luminescent (EL) display panel, a light emitting display device using the same, and a driving method thereof.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. The organic emitting cell includes an anode (e.g., indium tin oxide (ITO)), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies. Further, the organic emitting cell includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emission cells are classified as a passive matrix method, and an active matrix method using thin film transistors (TFTs). In the passive matrix method, anodes and cathodes cross (i.e., cross over or intersect) with each other, and lines are selected to drive the organic emission cells. In the active matrix method, a TFT is coupled to each ITO pixel electrode, and drives the line according to a voltage maintained by a capacitance of a capacitor coupled to a gate of the TFT. The active matrix method is further categorized, depending on formats of signals applied to the capacitor for establishing the voltage, as a voltage programming method or a current programming method.
The pixel circuit of the conventional voltage programming method has difficulties in obtaining high gray scales because of the deviations of the threshold voltage VTH and the carrier mobility, the deviations being caused by non-uniformity of a manufacturing process. For example, in order to represent 8-bit (i.e., 256) gray scales in the case of driving TFTs with the voltage of 3V (volts), the voltage gradation applied to the gate of the thin film transistor is less than 12 mV(=3V/256). Therefore, if the deviation of the threshold voltage of the TFT caused by the non-uniformity of the manufacturing process is 100 mV, it is difficult to represent high gray scales.
The pixel circuit of the current programming method achieves substantially uniform display characteristics when the driving transistor in each pixel has non-uniform voltage-current characteristics, provided that a current source for supplying the current to the pixel circuit is substantially uniform throughout the whole panel.
However, the pixel circuit of the current programming method has a long data programming time because of a parasitic capacitance component of the data line. In detail, the time (i.e., the data programming time) for programming the data on the current pixel line is influenced by a voltage state of the data line according to the data of a previous pixel line; in particular, the data programming time is further lengthened when the data line is charged with a voltage which has a large difference with the target voltage (i.e., the voltage corresponding to the current data). This phenomenon increases as the gray level becomes lower (i.e., near black.). FIG. 1 is a graph showing variations of data programming times per gray in the conventional light emitting display device. The times t1 to t7 in FIG. 1 represent the data programming times, and the legend on the right of the graph indicate gray levels of the data programmed to the pixel circuit coupled to the previous pixel line.
For example, when the gray level of the data programmed to the pixel circuit coupled to the previous pixel line is “8” and the gray level of the data to be programmed to the pixel circuit coupled to the current pixel line is 8 (i.e., a point where a curve meets the horizontal axis), the time needed for data programming is almost “0” since there is no difference between the voltage state of the data line and the target voltage.
However, as the gray level of the data to be currently programmed becomes farther from the gray level of 8, the time needed for data programming is increased since the difference between the voltage state of the data line and the target voltage increases.
The time needed for data programming is inversely proportional to the magnitude of the data current for driving the data line, and hence, when the gray level is lowered, the data current for driving the data line is reduced, and the data programming time is steeply increased. That is, as can be seen in FIG. 1, as the gray level becomes lower (i.e., near the black level), the data voltage is changed to a large voltage range with a low current, and the data programming time is increased.